Damage to Hearing

The most frequently occurring injurious effect of noise is damage to hearing. Noise energy of grossly high intensity can cause direct physical injury to other parts of the body but such effects are rarely found because levels of sufficient magnitude and duration to directly abrase the skin surface or to cause destructive vibration of the tissues or skeletal structures are rarely encountered in our current environment. Such effects are worth mentioning, however, for they emphasize the vibratory (i.e., purely mechanical) nature of stimulation by noise. Even when sound is not experienced as a sensation of vibration it is nevertheless acting in this purely mechanical way directly on the surface of the internal ear receptor system. Vibration of air particles (which is the major environmental medium for transmission of acoustic energy from source to listener) is transformed into vibration of fluid in the inner ear, via the ear-drum and the series of bones in the middle-ear. The middle-ear system acts to facilitate transmission of energy from aerial to fluid media. Furthermore, the middle-ear muscles (particularly the stapedius muscle which acts on the innermost bone of the middle-ear chain) reflexly operate at high intensity levels to cause attenuation of vibrations to the inner ear. In man, such attenuation is almost exclusively at mid to low frequencies (Borg, 1972).